A number of beautiful stamps issued in various countries of the world are known. Those stamps have peculiarities of the countries which issued them and are varied in design, size, constituent, and the like.
In recent years, the number of pieces of mail has been increasing and thus, the number of pieces of mail to be processed daily at post offices, etc. has been increasing. In order to facilitate work to process the mail, mechanization of this work has been developed, particularly at the post offices. For instance, there has been used a detecting system for distinguishing such mail depending upon the stamps put on them, whereby facilitating the work to process them at the post offices. As such system, there are known a system of distinguishing mail by detecting colors printed in the stamps put on the mail, a system of distinguishing mail by detecting emission from fluorescent pigments such as inorganic fluorescent pigments contained in the stamps put on the mail, and the like, which facilitate the processing work for the mail at the post offices such as those for distinguishing the mail whether or not they have proper stamps thereon. Of these systems, the system of processing the mail with the utilization of emission from the stamps put thereon has been mostly developed.
Those stamps containing fluorescent pigments which are commonly referred to as luminescent stamps are generally prepared by a method in which a mark, an image or the like, is printed on the surface of a sheet for stamps with a printing ink incorporated with an inorganic fluorescent pigment, using a gravure printing machine. Upon printing said printing ink on said sheet in this case, a doctoring blade is usually used to remove an excessive portion of the printing ink disposed on the gravure roll of the gravure printing machine. In this process there often occur problems that since the printing ink contains the inorganic fluorescent pigment in an insufficiently dispersed state, minute scratches including minute claw-like lines, etc. are often caused on the gravure roll and those minute scratches are transferred onto the sheet to provide undesirable prints which are defective not only in appearance but also in printed state.
In order to eliminate the above problems, there has been made a proposal that instead of incorporating the aforesaid inorganic pigment into the ink, said inorganic pigment is incorporated into the coating layer of a sheet for stamps. Such sheet for stamps is prepared by applying a coating composition containing an inorganic fluorescent pigment onto a base sheet (namely, substrate) by a coater to form said coating layer on the surface of the base sheet. However, as well as in the above case using the inorganic fluorescent pigment-containing ink, there often occur problems also in this case that grit of the inorganic fluorescent pigment provides negative influences to cause minute scratches. In addition to this, the inorganic fluorescent pigment is harder than other coating pigments such as calcium carbonate, and the like, and because of this, it causes damage to a cutting blade upon cutting a stamp product into stamp pieces or upon die-cutting a stamp product applied with tack treatment into stamp pieces, respectively by said cutting blade.